Almiron was injured in the first half of Atlanta's 2-0 win over the Montreal Impact on Sunday. He was replaced by Julian Gressel in the 17th minute.
The timeline means the Paraguay international likely will miss at least three of Atlanta's final five games of the regular season, as well as the final two CONMEBOL matchdays in World Cup qualifying.
The 23-year-old could return by Oct. 15, when his side visits the New York Red Bulls, and be back for the start of the playoffs in late October.
Almiron has scored nine goals in 29 games for Atlanta. He also leads the expansion club with 13 assists.
Atlanta (14-8-7) is sitting nicely entering the final stretch of the regular season, currently in third place in the Eastern Conference.